Herpesvirus infections are postulated to be important co-factors in the progression of HIV infection from quiescence to AIDS. Many children with HIV infection have never had chickenpox and therefore are at high risk to develop chickenpox which is the primary infection with varicella-zoster virus (VZV), a member of the herpesvirus family. The investigators plan to determine whether or not there is progression of HIV disease in children who develop varicella in comparison to those who do not, by studying a cohort of HIV-infected children who are susceptible to chickenpox. The investigators will prospectively enroll 100 HIV-infected children who are relatively asymptomatic and who have no prior history of varicella. During the course of study it is anticipated that about half of the children will develop varicella. The investigators will use laboratory means to document the varicella infection and will determine what percent of children develop chronic varicella and/or zoster and relate this to measurement of their immune function. The investigators will determine whether the VZV isolated from these children is sensitive to acyclovir, both before and after treatment with this medication. All children will be followed longitudinally for their clinical status, indications of multiplication of HIV, immune function and immune responses to VZV. The latter will include specific antibody levels and cell-mediated immunity related to lymphocyte stimulation against VZV antigen and antibody-dependent cellular cytotoxicity (ADCC0 to VZV. The investigators will compare the clinical course of children who do and do not develop varicella by comparing their growth, development of new symptoms related to HIV infections such as fevers, lymphadenopathy, parotitis, neurologic and developmental status, and occurrence of AIDS-defining opportunistic infections. The investigators will also compare indicators of HIV infection status: number of circulating CD4+ cells, p24 antigen levels, virus load (by plasma viremia and quantitative PCR) and serum immunoglobulin levels. Thus, the investigators will determine whether varicella worsens and underlying HIV infection by comparing the course of those children who develop chickenpox with those who do not develop it. If it can be found that VZV does not adversely affect HIV infection, it will mean that varicella vaccine trails can be undertaken in children with quiescent HIV infection. If the opposite is found, liberal use of antiviral therapy against VZV should be considered when there is clinical illness or tendency to recurrence.